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Quasars are a type of Active Galactic Nucleus that inhabit the centres of some galaxies. They are among the most energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way in radiation across the electromagnetic spectrum from X-rays to the far infrared with some quasars also being strong sources of radio emissions and of gamma-rays.

Quasars are powered by accretion of material into supermassive black holes in the centre of galaxies. It is thought that most galaxies, including the Milky Way, have gone through an active stage, appearing as a quasar or some other class of active galaxy, and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.

Does this mean that the amount of radiation on a planet in a quasar galaxy would prevent the development of the carbon-based life similar to what we have on Earth?

The answer to this question doesn't seem to address the issue

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    $\begingroup$ The lack of a definition (in scientific terms) for "life" makes answering questions like this a problem. We only have one set of examples of what carbon-based life looks like and those are related (all life on Earth) and formed in the same environment. No much to go on. $\endgroup$ Mar 31, 2022 at 22:41
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    $\begingroup$ Given @StephenG 's comment, it might make the question more answerable to ask "If the black hole at the center of the Milky Way galaxy was replaced with an AGN, would it cause harm to life on earth?" I suspect the answer to that question is no, except possibly if the solar system actually passed through the AGN jets. $\endgroup$
    – antlersoft
    Mar 31, 2022 at 23:19
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    $\begingroup$ Since dust is opaque to bluer (and thus more harmful) photons, I imagine one’s orientation in the galaxy would matter a great deal as well $\endgroup$
    – Justin T
    Apr 1, 2022 at 0:55

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There is some evidence that the Milky Way black hole has been active in the relatively recent past. Joss Bland-Hawthorn (et.al) describe large-scale ionization cones in the Galaxy that they say indicate a "Seyfert explosion ($10^{56}-10^{57}$ erg) that occurred in the Galactic Centre a few million years ago."

If this is correct, then life can and did develop in a galaxy that at times hosted an active galactic nucleus.

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  • $\begingroup$ +1 But would that have been a quasar? The most powerful AGN are classified as quasars, but what is the cut-off level? Is 1056−1057 erg sufficiently energetic to qualify as a quasar? BTW, I suspect the answer to the original question is 'yes' since most of the radiation from a quasar seems to be directed down the jet, rather than in the plane of the galaxy. $\endgroup$ Apr 3, 2022 at 10:49
  • $\begingroup$ My understanding is that, yes, that would be considered a quasar, from a certain point of view. $\endgroup$
    – James K
    Apr 3, 2022 at 11:12
  • $\begingroup$ If the Seyfert explosion wasn't considered a quasar, then there was also another recent extreme period in our galaxy that probably was. It was the event that created the Fermi Bubbles, which to my understanding is a separate event to the one that James K mentioned. To create structures like ours that span tens of thousands of lightyears, you're likely going to need a lot more than just an ordinary Seyfert AGN. That one was probably full-a blown quasar. This took place 6m years ago, while the planet was inhabited. Obviously, we're still here. $\endgroup$
    – 4NT4R3S
    Jan 9 at 18:50
  • $\begingroup$ Regardless, I do believe that 10^56 - 10^57 ergs would be a quasar - quite a powerful one actually. $\endgroup$
    – 4NT4R3S
    Jan 9 at 19:00

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